Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
To measure the position of a first component that moves with reference to a second component, an encoder typically uses one or more tracks on a carrier in which each track consists of a series of alternating dark and light bands that are viewed by a detector that outputs a digital value depending on whether the band currently being viewed is light or dark. The track is affixed to one of the components and the detector is affixed to the other.
Encoders can be divided into two broad classes. An incremental encoder typically utilizes a single track that is viewed by a detector that determines the direction and the number of bands that have passed the detector since a reference mark was detected. The position is determined by incrementing and decrementing a counter as each band passes the detector. The counter is reset when a reference mark is detected.
An absolute shaft encoder typically utilizes a plurality of tracks. The code pattern on each track has bands of different widths from the code pattern on the other tracks. An N-bit binary encoder typically utilizes N such tracks, one per bit. In addition to requiring a much larger number of detectors and a more complex code pattern carrier, an absolute encoder requires that the detectors for the various tracks be aligned with respect to one another, which further increases the cost of such encoders relative to incremental encoders.
While incremental encoders are less expensive than absolute encoders, incremental encoders are subject to errors that are often unacceptable. For example, if the circuitry fails to count a transition from a light to dark band, the counter, and hence, the position measurement will be in error until the counter is reset the next time the reference point is detected. Absolute encoders, in contrast, can be in error for at most one band of the track having the finest resolution. Hence, absolute encoders are preferred in many applications in spite of the additional cost associated with such encoders.
As the size of the mechanical systems that utilize encoders decreases, the size of the encoders must also decrease. Decreasing the size of an absolute encoder presents a number of challenges. One factor that limits the minimum size of an absolute encoder is cross-talk between the detectors used on the various tracks. Each track in the encoder is illuminated with a light source. The light from the illuminated track is imaged onto a corresponding photodetector that determines whether the band currently being viewed is light or dark. The light that strikes the detector consists of light that is reflected from the code bands of the track corresponding to that detector as well as light from an adjacent track that is scattered into the detector due to imperfections in the optical system and code carriers. This scattered light forms a background that reduces the signal-to-noise ratio of the detector, and hence, can lead to errors in the measured position. As the code bands are reduced in size in an effort to reduce the size of the encoder, the light available from a track decreases, since the size of the bands must be reduced. In addition, the distance between the tracks decreases, which, in turn, reduces the buffer space between the tracks that protects each detector from scattered light from a neighboring track. Both of these factors lead to reduced signal-to-noise ratios.
The cross-talk problem is particularly acute in reflective encoders. In a reflective encoder, each track consists of a series of reflective and absorptive bands. Light is reflected from the reflective bands into the detector associated with the track. While the absorptive bands can be made nearly ideal by utilizing a hole in the code strip for the absorptive bands, the reflective bands are less than ideal. Ideally, the reflective bands are perfect mirrors. However, in practice, the mirrors have imperfections. In addition, debris accumulates on the surface over time. These factors result in a surface that scatters some portion of the light incident on the surface. Some of the scattered light falls on the detectors corresponding to the adjacent tracks.
Finally, as the size of the encoder decreases, the problems associated with aligning the various detectors increases. Hence, the cost of assembly increases.